Many industrial work vehicles, particularly electric lift trucks, operate on 36 or 48 volt batteries for energizing the traction motor. Some electric vehicles operate on 72 volt batteries. In such vehicles, the battery voltage is too high for use as the power supply for auxiliary devices such as the vehicle lights, horn, solenoid hydraulic valves, and the like. Such vehicles need an auxiliary power supply of lower voltage, preferably of 12 volts, because many standard automotive components including lamps and horns can be used and are available at lower cost than high voltage components.
In the prior art, electric work vehicles have been provided with an auxiliary power supply having a voltage lower than the battery voltage. In one such auxiliary power supply a dropping resistor is connected in series with the battery to provide a lower voltage for the auxiliary components. This arrangement is wasteful of power and the heat given off from the resistors may be objectionable. Also, in the prior art, it is known to tap the main battery to obtain a reduced voltage supply; however, this technique is not acceptable because uneven use of the battery cells is detrimental to the battery. Electronic power supplies are also known for reducing a higher DC voltage to a desired value. However, such power supplies are costly, especially where a large amount of power is required. Also, the electronic power supply requires additional compartment space on the vehicle.
For applications requiring relatively high power, it is known to obtain DC-to-DC voltage conversion by use of a rotary transformer. Such a transformer is shown in the Ball U.S. Pat. No. 693,098 and comprises a dynamoelectric machine having an armature driven by a separate motor. The armature winding is connected with a commutator having a set of primary brushes connected with a DC voltage source. A set of secondary brushes coacts with the commutator to derive an output voltage from the rotary transformer.
A rotary transformer is also described in the Middlebrook U.S. Pat. No. 3,875,495. The transformer of the Middlebrook patent comprises a dynamoelectric machine having a set of primary brushes connected with a voltage source which energizes the machine as a DC motor. Two sets of secondary brushes are connected in parallel to the output terminals for producing an output voltage of the transformer. The two sets of secondary brushes are variably positioned in opposite angular relation to the primary set of brushes by means of a manual or an automatic servo control.
A general object of this invention is to provide an improved auxiliary power supply for an electric work vehicle.